The invention relates to a contact lens casting mould comprising two casting mould halves, a female part having a concave shaping surface and a male part having a convex shaping surface, which, when joined together, define a closed volume the bounding surfaces of which are formed by the two shaping surfaces.
Contact lenses are made from a wide variety of transparent materials. In conformity with contact lens materials used up to now a distinction is made between so-called hard contact lenses, for example consisting of polymethylmethacrylate (PMMA), and soft swellable contact lenses, for example those containing poly-2-hydroxyethylmethacrylate (P-HEMA). More recently, so-called hard/flexible lenses have also been made. For these, materials having a high oxygen-permeability, such as, for example, fluorinated polymers, are preferably used. Hard/flexible contact lenses of that kind are usually referred to also as RGP contact lenses, RGP standing for Rigid-Gas-Permeable.
Up to now, hard contact lenses and RGP contact lenses have customarily been produced from moulded lens blanks by shaping processes involving removal of the material, such as turning on special lathes or by laser machining. In these processes, the geometries of the front and rear surfaces, for example spherical, elliptical, generally aspherical, single-curved or multi-curved, for monofocal, bifocal, multifocal or toroidal contact lenses of the desired focal power are produced. In the same shaping step, but often also thereafter, elements for stabilising the lens on the eye, such as a prism ballast, slab-off zones, etc., can also be provided. Thereafter, the surfaces and the edge region of the contact lenses have to be generally polished. Overall, the manufacture of hard/flexible contact lenses requires a large number of very high precision production stages and is very complicated and expensive.
One way of simplifying the manufacturing process is to use a one-surface casting mould method. In this method, the front or rear surface of the contact lens is produced by pouring a mixture of monomers into a casting mould having a defined shaping surface and subsequently polymerising. The second surface of the lens, which is still in the rough state, must again be shaped by removing material. The additional polishing steps and especially the machining of the edge also still need to be carried out. The many production stages increase the production costs enormously and, in addition, it is difficult to guarantee the reproducibility of the manufacturing process.
So-called soft contact lenses also were originally manufactured mainly by the described shaping process involving removal of the material or by the described one-surface casting mould method. Such soft contact lenses achieve their final shape and their high flexibility as a result of subsequent hydration, the lenses absorbing about 30%-80% and more water, depending on the lens material used. Over the course of time, these manufacturing processes for soft contact lenses have been modified, and attempts have been made to provide so-called one-step manufacturing processes in which the contact lenses and, especially, their front and rear surface geometries are produced in one casting step. The casting moulds used for this are usually in two parts and consist of a male part and a female part having appropriately formed shaping surfaces for the geometries of the front and rear surfaces of the contact lens. In the joined state, these shaping surfaces define a specific volume which is filled with a liquid monomer in order to make a contact lens.
A process of that kind is described, for example, in GB-A-2,216,065. In that process, contact lens pre-shapes are produced in special moulds. These contact lens pre-shapes ideally already have the desired geometries of the front and rear surfaces, but machining of the edge of the contact lens is carried out in a separate step. Both halves of the casting mould have stop elements which serve to determine the final position of the mould halves when they are joined together. These stop elements are constructed in such a manner that they are able to execute a kind of bending movement so as to permit a relative movement of the mould halves towards each other in order to compensate for shrinkage.
In order to ensure that the geometries of the shaping surfaces are transferred to the front and rear surfaces of the contact lens accurately and reproducibly, it must be ensured that the two joined mould halves do not come undone or move away from each other. In addition, a substantially constant closing force must be maintained in order that no undesirable and indefinable deformations of the casting mould halves occur. A certain pressure must also be exerted on the two mould halves to ensure that the two mould halves move towards each other in order to compensate for the polymerisation shrinkage of the contact lens. The joined casting mould halves are therefore gripped in clamps which press the halves together with a relatively precisely defined closing force. These clamps prove to be a disadvantage, however, during the polymerisation of the contact lens material. In many cases they prevent a locally uniform entry of the UV light or microwave radiation with which the polymerisation is effected.
WO 87/04390 attempts to counter that disadvantage by forming on one of the mould halves locating pins which slide into bores in the second mould half when the mould halves are joined together. The protruding locating pins are then compressed in the manner of rivets by heat-deforming. Apart from the fact that this method of fastening the two mould halves scarcely allows accurate control of the closing force, after polymerisation of the contact lens it is awkward to separate the two mould halves from each other.
Similar considerations apply to processes in which the two halves of the casting mould are welded to each other at their edges. In this case also, the welded points have to be broken open again in an awkward manner in order to remove the contact lenses from the mould.
The process described in LU-A-59,438 in connection with the manufacture of window frames and the like, in which the position of two mould halves is fixed by a locking groove, is unsuitable for the manufacture of contact lenses. The two mould halves are locked with regard to their position relative to each other inter alia by locking members and levers, with the result that any relative movement between the mould halves is no longer possible. This, however, also prevents compensation of the shrinkage during polymerisation of the contact lens. The contact lens would become detached from the shaping surfaces and would in all probability be unusable.
There is therefore the problem of providing a contact lens casting mould with which the disadvantages described above do not arise. In particular, the casting mould halves are to be capable of being joined together easily and reliably and separated from each other again with equal ease. When the casting mould halves are joined together, the applied closing force is to be maintained as well as possible. Furthermore, the pre-requisites for manufacturing also hard and hard/flexible contact lenses in a one-step manufacturing process are to be provided. In particular, it is to be possible to produce the front and rear surface geometries and the edge of the contact lens in the casting mould during the actual casting process, substantially without further finishing. The desired contact lens is to be in its final form as soon as it has been removed from the mould. In addition, the casting mould is to be inexpensive and simple to manufacture and is to provide a simple means of storing and transporting the contact lenses. Furthermore, the shaping surfaces of the casting mould halves are to be protected as far as possible from scratches and damage.